Osteoporosis is the net loss of bone mass and resultant structural weakening due to an uncoupling of normal bone homeostatic mechanisms, so that osteoclastic activity remains inappropriately high. Recent studies suggest that osteoclast formation is regulated by at least two soluble factors, 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) and CSF-1 (3,4). We have recently demonstrated that the response of osteoclastic progenitors from osteopenic aged mice to 1,25(OH)2D3 and CSF-1 is deranged in vitro. Additionally, precursor cells isolated from aged, osteopenic animals appear to form osteoclast-like cells more readily under the influence of 1,25(OH2)2D3 in an in vitro system. This represents a potential connection between regulation of osteoclast formation by these factors and failure to maintain bone mass in age-associated osteopenia. Based on the hypothesis that the osteopenia of aging is caused by or contributed to by dysregulated (inappropriately high) osteoclast production, we propose that this dysregulation may be due, at least in part, to a defect intrinsic to the osteoclast progenitor cell itself: i.e., an altered response to 1,25(OH)2D3 and/or CSF-1 or alternatively to alterations in the levels of these factors, thus favoring osteoclast formation. By study of osteoclast production in vitro, exploring the role of CSF-1 and its receptor during osteoclastogenesis, and examining the action of 1,25(OH)2D3 in directing osteoclast production, we hope to gain insight into regulation of osteoclast formation and function. Gaining an understanding of how these processes may be altered with aging may provide a mechanism whereby age-associated osteopenia occurs in a murine model. The following specific studies are planned: 1. To study the effects of 1,25(OH)2D3 and CSF-1 on osteoclast progenitor cell proliferation and differentiation in osteopenic and control animals. 2. To quantify the availability of the osteoclastic regulators, 1,25(OH)2D3 and CSF-1 and their receptors in osteopenic and control animals. 3. A. To examine baseline CSF-1 receptor synthesis and kinetics in osteopenic and control animals, and to determine the mechanism whereby baseline CSF-1 receptor expression is increased in osteopenic animal. B. To study alterations in molecular (mRNA transcription and translation) or kinetics of receptor maturation, cycling from intracellular pools and ultimate degradation which may account for the observation that osteoclast precursor cells derived from osteopenic animals fail to upregulate the CSF-1 receptor in response to 1,25(OH)2D3. The above proposed studies should provide insight into the roles of 1,25(OH)2D3 and CSF-1 in regulation of osteoclastogenesis, as well as delineating mechanisms which we hypothesize to be important in the dysregulation of osteoclast activity and the ultimate development of age- associated osteopenia.